The present invention relates to the measurement of x-ray tube voltage and, more particularly, to the measurement of tube voltage in a computed tomography (CT) imaging system.
In a contemporary computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within the X-Y plane of a Cartesian coordinate system, termed the "imaging plane." The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile.
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a "view" and a "scan" of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
The quality of the image produced by any x-ray machine, and particularly a CT system, is determined in part by the quality of the accelerating voltage applied between the x-ray tube anode and cathode. This voltage is commonly called the peak kilovoltage (KVp) and its value is dependant on the particular machine in which the tube is used. In mammography, for example, better tissue contrast is achieved with relatively low voltages of around 30 KVp, whereas conventional x-ray machines and CT systems employ higher voltages in the range of 80 KVp to 140 KVp. All x-ray machines are subject to errors and image artifacts caused by incorrect tube voltage. CT systems are particularly vulnerable to variations in tube KVp, since they rely on a known KVp to make corrections to the acquired data for phenomena such as beam hardening. Also, special procedures such as bone mineral densitometry require an accurate KVp to provide the desired image contrast reproducibility. The KVp stability (or absolute KVp value) of an x-ray machine may be degraded by such events as long-term component drift, or component stress produced by x-ray tube "spits". As a result, KVp recalibration is performed regularly by service personnel and is a very time consuming task.
Commercial instruments are available which allow measurement of KVp from differential filtration of the x-ray beam, but these instruments are expensive, inconvenient, and are not highly accurate. In addition, available instruments do not allow measurements without service personnel being present to insert the measurement device into the beam, and beam measurements may not be made while the scanner is being used on a patient.